At present the only industrial method of producing hydrogen fluoride is sulfuric acid decomposition of mineral raw material, namely, fluorspar. As the end product either hydrofluoric acid is obtained (38-41% aqueous solutions of hydrogen fluoride) or non-aqueous hydrogen fluoride, which depends on the production technology. The composition of the end product depends on the quality of the initial fluorspar. Fluorspar with a calcium fluoride content less than 45 wt. % is not used for producing hydrogen fluoride. To obtain non-aqueous hydrogen fluoride, fluorspar is needed with a content of calcium fluoride no less than 95-97 wt.%.
The technological scheme of the known method consists of the following stages:
(a) decomposition of fluorspar with an acid at a temperature of 180.degree.-250.degree. C. according to the reaction: CaF.sub.2 +H.sub.2 SO.sub.4 .fwdarw.CaSO.sub.4 +2HF
(b) purification of the gases of fluorspar decomposition from impurities of fluorspar particles, sulfur- and phosphorus-containing compounds, and other components;
(c) absorption of purified gases with water in order to obtain solutions of hydrofluoric acid (if production of non-aqueous hydrogen fluoride is not envisaged);
(d) condensation of purified gases. If non-aqueous hydrogen fluoride is the final product of the production, purified gases of fluorspar, decomposition are subjected to condensation for extracting hydrogen fluoride by cooling down to a temperature of minus 5.degree.-10.degree. C. The obtained condensate of hydrogen fluoride (the so-called "crude") is purified from the main impurities (H.sub.2 O, H.sub.2 SiF.sub.6, H.sub.2 SO.sub.4) by rectification; hydrogen fluoride is obtained as a commercial product in non-aqueous liquid state.
However, the cost of hydrogen fluoride, especially in non-aqueous liquid state, obtained by following the known technology is rather high, which limits the field of its application in various branches of industry. In addition, this method requires considerable amounts of raw materials, chemicals, and energy, highly resistant anticorrosive materials, and is technologically complicated. At the same time, as high-grade mineral fluorspar is being consumed, which means that its resources are reduced, the cost of hydrogen fluoride and criticality constantly grow. Therefore, new types of phosphorus-containing raw material are sought for and methods of processing it into hydrogen fluoride are being developed. A similar situation is observed in the production of silicon dioxide, especially in connection with imposing more stringent requirements on its purity in a number of industries (optics, electronics, luminophores, etc.).
Silicon dioxide is produced by various methods depending on the requirements as to its purity. Among them are: combustion of wastes of agricultural production, treatment of natural sands, oxidation of silicon tetrachloride in plasma burners, treatment of silicic acids isolated in the utilization of effluent gases in the production of phosphorus-containing mineral fertilizers, etc.
As is known, production of phosphorus-containing mineral fertilizers in the chemical industry is based on sulphuric-acid decomposition of apatites and phosphorites containing fluorine-ion in amounts of 2-5 wt.%. Almost the whole amount of fluorine-ion passes from raw material into a gaseous phase and is removed as a mixture of hydrogen fluoride with silicon tetrafluoride from the production process together with the effluent gases.
Utilization of fluorine-ion in the effluent gases from the production of phosphorus-containing fertilizers is performed at present mainly to avoid pollution of the environment with various fluorides. The utilization products are aqueous solutions of various fluorine salts of fluosilicic acid, the need for which is rather limited. It has been established that fluorine resources in superphosphate raw material amount to 1000 millions of tons (on a world-wide scale) as compared to 30 min. tons in fluorspar. In spite of the fact, that trapping of the fluorine-ion from the effluent gases of superphosphate production reaches 98%, utilization of this ion does not exceed 40-50 wt.%.
Fluorine-containing effluent gases of superphosphate production contain, mainly, such fluorides as silicon tetrafluoride and hydrogen fluoride; they are promising as a source of not only fluorine, but silicon as well. Various methods of producing hydrogen fluoride from silicon tetrafluoride have been proposed, taking into account considerable amounts of fluorine (in the form of silicon tetrafluoride and hydrogen fluoride) in the effluent gases from superphosphate production. Particular interest in processing silicon tetrafluoride is also determined by the possibility of complex processing of raw material and production of not only hydrogen fluoride but silicon dioxide as well. Such technology will ensure quality and cost of silicon dioxide which will be beyond competition with the currently employed methods of producing silicon dioxide from wastes of agricultural productions, from sands and such critical products of chemical industry and metallurgy as silicon tetrachloride, ferrosilicon, etc.
Methods of processing silicon tetrafluoride contained in the effluent gases into hydrogen fluoride and silicon dioxide can be classified into two groups, namely, direct and indirect.
Indirect methods are based on extracting silicon tetrafluoride from the effluent gases of superphosphate productions by absorption with water or aqueous solutions of alkali and salts, or by adsorption with solid salts with subsequent processing of the trapped products into hydrogen fluoride. Fluosilicic acid and silica gel (hydrated silicon dioxide) are the products of silicon tetrafluoride absorption with water; otherwise the products include aqueous or non-aqueous salts of fluorides or silicon fluorides of, mainly, alkali or alkali-earth elements or ammonium.
Thus, it is proposed in U.S. Pat. (No. 2,819,151 of 1958, No. 3,087,787 of 1963, and No. 3,551,098 of 1970) to sorb silicon tetrafluoride and hydrogen fluoride in the effluent gases from superphosphate production on sodium fluoride. In this case the following reactions take place: EQU SiF.sub.4 +2NaF.fwdarw.Na.sub.2 SiF.sub.6 EQU HF+NaF.fwdarw.NaF.HF
Extraction of hydrogen fluoride from a mixture of solid salts of sodium bifluoride and silicon fluoride is proposed to be run with heating up to a temperature 320.degree.-360.degree. C.; the remaining mixture of silicon fluoride and sodium fluoride can be used either for extracting silicon tetrafluoride by heating the mixture up to 450.degree.-500.degree. C., or hydrogen fluoride by treating the mixture with sulphuric acid in a way similar to the decomposition of fluorspar.
In U.S. Pat. Nos. 3,218,124, 3,218,125, 3,218,126, 3,218,128 of 1965 and in Inventor's Certificate of the USSR No. 159.806 of 1964 it is proposed to trap silicon tetrafluoride from the effluent gases of superphosphate production with water. Fluosilicic acid thus obtained is filtered from silica gel; purified acid having a concentration of 6-12 wt.% is subjected to dehydration with concentrated sulphuric acid (concentration of sulphuric acid is no less than 90-92%). Upon dehydration fluosilicic acid decomposes to silicon tetrafluoride which is then returned into the process, and hydrogen fluoride sorbed by sulphuric acid. Hydrogen fluoride is extracted from a mixture of sulphuric and fluosilicic acids either by heating above 150.degree. C. or by extraction with hexane or trinonylamine. The reactions taking place are described by the following equations: EQU 3SiF.sub.4 +3H.sub.2 O=2H.sub.2 SiF.sub.6 +H.sub.2 SiO.sub.3 EQU 2H.sub.2 SiF.sub.6 +2H.sub.2 SO.sub.4 =2SiF.sub.4 +2[H.sub.2 SO.sub.4.2HF] EQU 2[H.sub.2 SO.sub.4.2HF]=2H.sub.2 SO.sub.4 +4HF
Besides, in British Patent (No. 963,089 of 1964), U.S. Patent (No. 3,195,979 of 1965) and in patents of other countries it is proposed to treat the solutions of fluosilicic acid, obtained after sorption of silicon tetrafluoride, with water, with ammonia solution and then to separate the products of acid decomposition: EQU 3SiF.sub.4 +3H.sub.2 O=2H.sub.2 SiF.sub.6 +H.sub.2 SiO.sub.3 EQU 2H.sub.2 SiF.sub.6 +12NH.sub.4 OH=12NH.sub.4 F+2H.sub.2 SiO.sub.3 +6H.sub.2 O
The solution of ammonium fluoride, filtered from silica gel, is evaporated up to the formation of ammonium bifluoride from which hydrogen fluoride is removed at a temperature of 150.degree. to 170.degree. C.: EQU 2NH.sub.4 .fwdarw.NH.sub.3.2HF+NH.sub.3 EQU NH.sub.3 2HF.fwdarw.NH.sub.4 F+HF
In patent literature there is a considerable body of examples similar to the above-cited on indirect treating silicon tetrafluoride. But they all have the main disadvantages complicating their practical application: the necessity of operating with large volumes of aqueous solutions of weak acids and salts, separation of silica gel and fluorine-containing compounds by filtration, complexity of processing the obtained fluorine compounds into hydrogen fluoride (in the majority of cases similar to decomposition of fluorspar with sulphuric acid), and impossibility of using silica gel for producing a side product (silicon dioxide) because of a considerable amount of impurities in it.
Therefore, attempts were made to simplify the process and to develop direct methods of processing silicon tetrafluoride. A method has been proposed, according to which silicon tetrafluoride, without being separated from gases of superphosphate production, is subjected to hydrolysis with steam at 500.degree.-900.degree. C. (U.S. Pat. No. 3,087,787). As a result of hydrolysis, a mixture is formed, consisting of silicon dioxide and hydrogen fluoride, according to the reaction: EQU SiF.sub.4 +2H.sub.2 O.fwdarw.4HF+SiO.sub.2.
It is proposed to separate silicon dioxide from the mixture by filtration on cermet or ceramic filters at a temperature no less than 175.degree.-220.degree. C. But only 50% of silicon dioxide is separated by this method: the rest of silicon dioxide together with hydrogen fluoride is passed through sodium fluoride. Sorption of a mixture HF+SiO.sub.2 on sodium fluoride leads to the formation of a mixture of solid silicon fluoride and sodium bifluoride: EQU SiO.sub.2 +6HF+4NaF.fwdarw.2[NaF.HF]+Na.sub.2 SiF.sub.6.
Hydrogen fluoride is separated from this mixture by heating to 300.degree.-350.degree. C.; the remaining part is either discharged or processed by decomposition with sulphuric acid.
It is also proposed to burn the effluent gases from superphosphate production in a stream of oxygen or unsaturated hydrocarbons at 420.degree.-1700.degree. C. (U.S. Pat. No. 3,110,562 of 1963). As a result of the formation of water vapors and their presence in a gas flow at high temperatures, hydrolysis of silicon tetrafluoride takes place, giving hydrogen fluoride and silicon dioxide contaminated with silicon carbide. Separation of this mixture is proposed to be performed by filtration on filters, after which hydrogen fluoride is either trapped upon cooling with water or condensed with liberation of non-aqueous hydrogen fluoride.
In spite of the evident simplicity of direct methods of processing silicon tetrafluoride contained in the effluent gases, in these methods such major disadvantages are inherent as high temperatures of the process, the need for corrosion-resistant materials with respect to water vapors and hydrogen fluoride, complexity of the equipment and of the process of separating the mixture of silicon dioxide and hydrogen fluoride.
The authors of the present invention, by analyzing the chemical aspects of silicon tetrafluoride hydrolysis with water at room and elevated temperatures, have established that even at room temperature silicon tetrafluoride is hydrolyzed with the formation of silicon dioxide and hydrogen fluoride which, interacting with each other, form fluosilicic acid. Consequently, the end products of silicon tetrafluoride hydrolysis with water under ordinary conditions are silicon dioxide and fluosilicic acid but not hydrogen fluoride: ##EQU1##
Thus, all the hitherto proposed methods for processing silicon tetrafluoride into hydrogen fluoride and silicon dioxide have technical and economic parameters much inferior to those of the method of producing hydrogen fluoride from fluospar, and therefore, have not found practical application.